The present invention relates to a system and a method for generating periodic and/or quasi-periodic pattern on a sample using space-invariant interference lithography.
Periodic and/or quasi periodic structures find extensive use in many areas of micro- and nano-technology. Optical gratings, diffraction lenses, patterned magnetic media for data storage, integrated electronic circuits, sensors and display devices are some examples where periodic structures are used. Other application areas using periodic structures include sub-wavelength optical elements, templates for nano-imprint lithography, templates for guided self assembly, templates for crystallization, arrays of nanowires, nanodots, process development for the next generation lithography technologies, in particular Extreme Ultraviolet Lithography (EUVL), catalysis and field emission substrates.
These uses are expected to grow significantly as novel properties of materials with nanometer scale dimensions are exploited in such or other new applications yet not known. Lithographic methods are usually used to fabricate such periodic structures. However, currently available lithographic methods for fabricating patterns with periods less than 100 nm are too costly for widespread exploitation.
Interference lithography (IL) is a known technique for creating periodic structures. Lasers in the visible or ultraviolet regions and synchrotrons in the extreme-ultraviolet (EUV) range are employed as light sources in IL. Periodic, quasi-periodic, curved one and two-dimensional patterns can be produced with IL. Quasi-periodic patterns are described in H. H. Solak, C. David, J. Gobrecht, Fabrication of High-Resolution Zone Plates with Wideband Extreme-Ultraviolet Holography, Appl. Phys. Lett. 85, 2700 (2004). Curved patterns are described in EP 03003 392.
Most IL methods require light sources with a high degree of spatial and/or temporal coherence. Lasers operating in the visible and ultraviolet region often possess one or both of these properties and, therefore, they are used in the vast majority of IL applications. An achromatic IL method with relaxed requirements with respect to these two properties has been described in T. A. Savas, S. N. Shah, M. L. Schattenburg, J. M. Carter, H. I. Smith, Achromatic Interferometric Lithography for 100-nm-Period Gratings and Grids, J. Vac. Sci. Technol. B 13, 2732 (1995). However, this technique requires very high power from the source as the beam is diffracted by two consecutive gratings with limited efficiency. Moreover, the depth of focus of the obtained pattern is limited by the spatial coherence of the source.
A related technique to IL is Spatial Frequency Multiplication (SFM), where the substrate to be patterned is placed behind a transmission optical grating (mask), as described in U.S. Pat. No. 4,360,586. In a plane parallel to the grating and at certain distances away from the grating, the intensity of light has a periodicity that is a multiple of the periodicity of the mask. In SFM, the sample is placed in a plane where the desired intensity distribution exists and the intensity distribution is recorded in a photo-sensitive film. In this way, new gratings with spatial frequencies that are multiples of the original diffraction grating can be formed. In the described technique, the intensity distribution depends sensitively on the distance from the grating. Therefore, very precise positioning and alignment of the sample to be patterned with respect to the mask (narrow field) is required. This “depth of field” limitation becomes much more restrictive as the period of the mask is reduced. Moreover, this limitation excludes the use of the technique for creating patterns on substrates with topographical features that are larger than the depth of field.